Treatment of the bis-acetylicle zirconocene complexes (RCP)(2)Zr(C C-SiMe3)(2) (8a, R = CH3; 8b, R = tert-butyl) with HB(C6F5)(2) results in the formation of the five-membered organometallic zirconacycloallenoid products (9a, 9b). Both were characterized by X-ray diffraction, and the special bonding features of 9a were studied by DFT calculation. The overall reaction was followed by NMR experiments at variable temperature. Starting from 8a the reaction proceeds by alkynyl abstraction to give a zwitterionic [(MeCP)(2)ZrC C-SiMe3](+)[eta-(Ar2B)-B-F(H)C C-SiMe3](-) intermediate (12a), single crystals of which were obtained at -30 degrees C for characterization by X-ray diffraction, followed by a 1,1-hydroboration/sigma-ligand coupling sequence to give the new five-membered zirconacycloallenoid product 9a.